The plansifter is one of the few machines which, for nearly a hundred years, has withstood all attempts made by inventive ingenuity to replace it. Still utilizing the same basic technique, it remains unsurpassed for certain uses in mills, namely producing and grading individual products of grinding operations such as baking flour, semolina, etc.
The entire plansifter mechanism is given a horizontal circulating movement which not only allows the actual sifting work to be carried out but also conveys the product from sieve to sieve and to outlets. The mounting and drive of the plansifter can be of such a construction that the plansifter operates as either a freely oscillating apparatus or a positive drive device constrained in its movements.
One of the disadvantages of positive drive, particularly in large plansifters, is the provision of guides required for the positive drive. The mechanism necessarily includes a crank drive which has attendant lubrication and maintenance problems. In the case of positive drive sifters more oscillation forces are transmitted outward from the device than in a freely oscillating plansifter. It is desirable, therefore, that plansifters be designed so that they do not suffer damage when operational disturbances occur, even if the sifters remain in operation partly or completely filled with grinding products.
If the present market situation is considered, it will be noted that there has been a certain standardization of plansifters. Large plansifters which are usually constructed as freely oscillating sifters are, numerically, clearly the dominant machine. The second most frequently used are small plansifters constructed as positive drive sifters having only a single open stack of sieves.
Medium size sifters which usually utilize two sieve compartments or stacks are used relatively infrequently. Suggested reasons for this include high price, inadequate economic advantages, and constructional defects. It has been difficult, however, to specifically identify one predominant reason.
It is recognized in the technical world that the running-up and running-down periods of a freely oscillating plansifter not utilizing an outside drive shaft do not occur free of resonance. Using the formula w.sup.2 =g/l (wherein l is the length of the pendulum suspension) the angular velocity or resonance range can be estimated, and values are generally calculated, for example, where the range of the rotational speed of the unbalance is about 10-30 rpm. The operational speed of the unbalance in most cases is far above these values, but a freely oscillating plansifter has to pass through this critical resonance range when running-up and running-down.
In large plansifters resonance problems are greatly reduced since they have a large number of inlets and outlets or other points of connection securing the device to parts of the structure or building in which the device is housed. This construction has an inhibiting effect on any excessive outward throw of the plansifter.
The situation is rather different with a single stack sifter. Factors stimulating resonance are more pronounced than for large plansifters. For example, the mass of material being sifted in control sifting may be a much larger percentage of the total weight of the plansifter. Mechanical problems are also increased by fluctuating weights and center of gravity positions. In a single-stack sifter a compromise has to be struck between constructional design, access to replacement of sieves, and the arrangement of the unbalance. It has generally been found that, regarding forces such as those caused by resonance, compromise is very dangerous and often leads to serious damage. One solution has been to use the positive drive construction for small or single-stack sifters. To a degree, such construction allows the dangerous resonance ranges to be avoided or cut out in all working conditions. However, with a single-stack sifter, the number of stacked sieves has had to be limited to between 6 and 9 l since, if more were used, wobble or run out forces would prejudice operational safety and reliability. The number of possible separations in a single-stack sifter is usually between 2 and 6, and stacks of sieves whose overall surface is smaller are used.
Operational experience over many decades has proven the effects described above for various sized plansifters. Since the freely oscillating apparatus is not suitable for small sifters, and conversely positive drive has not been found very suitable for large plansifters, the engineer has been presented with the difficult task of deciding what design is best for the medium sized, two-stack sifter.